Known high performance wires generally include a functional core such as an electrical conductor or optical fiber, and one or more insulating and/or protective coatings. These coatings should be flexible and not too bulky, since wires are required in many cases to be of small diameter. Such small diameter, high performance wires may be formed by wrapping thin single or multilayer tapes, by extrusion or by a combination of these techniques.
Various types of polymers are known for use with wire and cable sheathing, such as polytetrafluoroethylene (PTFE) and polyetheretherketone (PEEK). PTFE has the advantage of being very tough as well as chemically inert, with a high softening point, low coefficient of friction and good electrical insulating properties.
PEEK has found increasing use in wire and cable sheathing since it has good flame resistance, being self-extinguishing with very low smoke. It also has good elongation, good flexibility in thin sections, such as films and good mechanical resistance to dynamic cut-through and scrape abrasion. It can however be susceptible to arc tracking, as well as reaction with acetone and strong acids.
EP-A-572 177 discloses a known electrical insulation laminate of porous PTFE and PEEK. The purpose being to provide a flexible electrical insulation material for air frame wire insulation which is lightweight with high mechanical strength, thermal resistance and chemical resistance and reduced dielectric constant.
There is a demand for wire and cable insulations that are resistant to fire as well as to very high temperatures. One way of imparting such flame resistance is to apply a coating comprising mica particles, typically platelets, dispersed in a polymer matrix. JP-A-2003100149 for example discloses the use of a dispersion of fine mica powder and glass frit in a silicone resin for coating fire resistant cables.
Mica can however add to the cost and accordingly there is a need to reduce the mica content of cable sheaths. For example, JP-A-2006120456 seeks to avoid the use of mica by combining a glass tape to impart tensile strength and dimensional stability with a silicone tape to impart heat resistance, electrical characteristics and adhesiveness.
JP-A-2000011772 discloses a fire resistant coating made with a cross-linked silicone rubber mixed with aluminium hydroxide and mica powder.
There is also a requirement for wires and cables of reduced diameter, which could be achieved by using sheaths of reduced thickness.
GB-A-2460686 discloses a wire having a functional core and a sheath including an inner flame proofing layer comprising mica particles dispensed in a polymer matrix such as silicone, a wrapped film of PEEK and an outer coating of PEEK or another polymer. In this way the required temperature and flame resistance can be obtained with mica confined to one layer of three, with a thickness of preferably not more than 100 μm.
WO8900757 describes a wire having good electrical and mechanical properties with an inorganic arc-control layer surrounding the conductor, an insulating layer comprising an aromatic polymer which surrounds the arc control layer and a secondary tracking control layer surrounding the insulating layer. The wire exhibits good resistance to dry arcing and wet tracking
There remains, however, a need for wires and cables which, in addition to good insulation and high temperature resistance, have improved mechanical properties, especially at high temperatures.